Techniques related to inkjet printing form a rich domain in terms of drop generators dedicated to the controlled production of calibrated drops.
One possible technology is the continuous inkjet family that requires the pressurization of ink in an ink reservoir enclosed in the print head to form a continuous liquid jet: the ink reservoir comprises particularly a chamber that will contain ink to be stimulated, and a housing for a periodic ink stimulation device. Working from the inside outwards, the stimulation chamber comprises at least one ink passage to a calibrated nozzle drilled in a nozzle plate: pressurised ink passes through the nozzle, thus forming an ink jet.
The jet is broken into droplets using a stimulation device, the function of which is to modulate the radius of the jet; this forced fragmentation of the ink jet is usually induced at a point called the drop break up point by periodic vibrations of the stimulation device located in the ink reservoir on the upstream side of the nozzle. Jet radius modulation is amplified under the action of the surface tension of the liquid. This physical phenomenon, widely used in industrial continuous jet printers, was initially described and modelled by Lord W S Rayleigh (<<On the Instability of Jets>>, Proceedings of the London Math. Soc. 1879; X: 4-13).
A variety of means is then used to select drops that will be directed towards a substrate to be printed or towards a recuperation device commonly called a gutter. Therefore the same continuous jet is used for printing or for not printing the substrate in order to make the required patterns.
Various stimulation techniques can be envisaged. For example, Electro-Hydro-Dynamic (EHD) stimulation described in U.S. Pat. No. 4,220,928 (Crowley) consists of applying a potential difference between an electrically conducting jet at ground potential and an electrode at variable potential; the electrostatic pressure at the jet surface deforms the jet and the modulation of the radius is amplified by capillary instability leading to breaking up the jet.
Another approach is thermal stimulation, for example described in U.S. Pat. No. 4,638,328 (Drake): there is an imposed disturbance of the radius (or velocity) controlled by a thermo-resistive element close to the nozzle. Recent industrial developments have been derived from the silicon technology to manufacture this type of thermal drop generator (for example see Kodak's patent US 2003/0222950). However, the body of the drop generator is made of silicon, a material known for its mechanical weakness and very mediocre chemical resistance particularly in an alkaline medium, which limits the nature of projected liquids. Furthermore, actuators produce heat and consequently the accumulation of heat can increase the temperature of the head, thus modifying the properties of the ink and the associated physical parameters (for example the viscosity and therefore the jet velocity). It is difficult to control this temperature rise, knowing that the electrical energy dissipated in the heating resistances depends on the pattern to be printed. Finally, the action created on the jet takes place in a single direction, since the heating resistance is only capable of increasing the temperature of the ink, and it is not possible to create a disturbance on the jet inverse to that caused by heating. This point limits the control accuracy of the drop formation process.
These two techniques (EHD & thermal) have the advantage of being inherently non resonant; the addressed/stimulated portion of the jet is perfectly defined and enables asynchronous production of different size drops or segments. The disadvantage of these techniques is their low efficiency, which requires the use of very strong electrical control levels, or the use of complementary physical phenomena to efficiently break up the jet.
Apart from these approaches, generation of drops with a constant mass and velocity at a fixed frequency in a single-jet system, is also described in U.S. Pat. No. 3,596,275 (Sweet), wherein the stimulation device is a piezoelectric actuator. The main advantages of these types of actuators are excellent control over the drop size; the high operating frequency; and the efficiency and lack of a direct electrical contact between the fluid and the actuator.
Such continuous jet printers may comprise several print nozzles operating simultaneously and in parallel, in order to increase the print surface area and therefore the print speed. The piezoelectric stimulation technique is broadly used for the design of multijet generators, for example with an actuator for a jet array like the one described in U.S. Pat. No. 3,373,437 (Sweet), or an actuator for each jet as described in WO 01/87616 (Marconi).